The invention relates to a brushless DC motor of the type used in spindle motors in disk drives and, in particular, to a hydrodynamic bearing for such spindle motors.
Disk drive systems have been used in computers and other electronic devices for many years for storage of digital information. Information is recorded on concentric memory tracks of a magnetic disk medium, the actual information being stored in the form of magnetic transitions within the medium. The disks themselves are rotatably mounted on a spindle, the information being accessed by means of transducers located on a pivoting arm which moves radially over the surface of the disk. The read/write heads or transducers must be accurately aligned with the storage tracks on the disk to ensure proper reading and writing of information. Thus the disks must be rotationally stable.
Electric spindle motors of the type used in disk drives conventionally rely on ball bearings to support a rotary member, such as a rotating hub, on a stationary member, such as a shaft. Ball bearings are known to wear parts, and, in time, increased friction will cause failure of the motor. In addition, ball bearings create debris in the form of dust or fine particles that can find their way into “clean” chambers housing the rotary magnetic disks which are driven by the motor. The mechanical friction inherent in ball bearings also generates heat, noise and vibration, all of which are undesirable in a disk drive motor.
Hydrodynamic bearings represent a considerable improvement over conventional ball bearings in spindle drive motors. In these types of bearing systems, lubricating fluid, either gas or liquid, functions as the actual bearing surface between a stationary base or housing and the rotating spindle or the rotating hub of the motor. For example, air or liquid lubricants, comprising oil or more complex ferro-magnetic fluids have been utilized in hydrodynamic bearing systems.
The advantages of hydrodynamic bearings over ball bearings include improved running accuracy, greater impact strength, and less noise development.
Spindle motors for data storage medium disks in which a motor shaft fixedly attached to a rotor is supported by means of a hydrodynamic bearing system are known in the art. A hydrodynamic bearing system according to the prior art comprises, for example, a bearing sleeve having a hollow cylindrical hole, which can be sealed on one side with a counter-plate. A motor shaft surrounded by a lubricating fluid, typically oil, is provided inside the bearing sleeve. One or more grooved patterns are provided on the inner surface of the bearing sleeve or on the outer surface of the motor shaft. These grooves generate hydrodynamic bearing pressure when the shaft rotates inside the bearing sleeve.
Furthermore, hydrodynamic bearings with axial track cap bearings in low-performance spindle motors are known in which the axial bearing forces in one direction are absorbed by supporting the bearing in its fulcrum on a counter-plate, while the axial opposing force is generated magnetically by, for example, the interaction between the rotor and the stator. However, these types of hydrodynamic bearings possess minimal axial rigidity in one direction. As a result, their use in hard disk drives is problematic because such applications require axial rigidity in both axial directions. On the other hand, hydrodynamic bearings with axial track cap bearings offer the advantage of having a very low level of friction loss and, consequently, a lower power consumption.
An example of a hydrodynamic bearings according to the prior art, as it is described above, is known from U.S. Pat. No. 4,934,836.